The extraterrestrials are just plain done with us. The Cold War has ended, and so has their fascination with our nuclear missile silos. So maybe they've just declared "mission accomplished," and gone away. That would be analogous to Charles Darwin's visit to the Galapagos Islands -- after he probed, bottled and cataloged some of the natives, he weighed anchor and withdrew. But here's another possibility.
Cosmologists propose that the universe was until recently a lifeless collection of particles. But they have ignored a critical component of the cosmos because they don't know what to do with it. This component, consciousness, is an utter mystery. How did inert, random bits of matter ever morph into Obama or Lady Gaga?